In recent years, there is a demand for a heat-resistant alloy suitably used to extend the life-span of a plastic working tool used in a high temperature environment, such as a die for hot extrusion, a piercer plug for a seamlessly produced pipe, and a hot runner nozzle for injection molding.
In recent years, friction stir welding (FSW) has been increasingly developed, and for a rotating tool used therefor, in order to extend friction stir welding's applicability, a material which is high in high temperature strength and room temperature hardness is increasingly developed.
Friction stir welding is a method in which a rotating tool is pressed against a welding portion of a metallic member to cause frictional heat, which softens a material to be welded and the material is caused to plastically flow and thus welded. Friction stir welding has already been increasingly practically used in welding a low melting point, soft material such as aluminum and magnesium and its applicability is extended. Currently, however, in order to contemplate application to a higher melting point and harder material to be welded, there is a demand for developing a tool which has a working life with enhanced high temperature strength and enhanced wear resistance.
This is because in FSW when a tool is used to cause frictional heat to soften a material to be welded, in general, the tool's temperature can increase to reach around 70% of the material's melting point, although there is a difference depending on the welding condition(s) and the material to be welded. In other words, for aluminum having a low melting point, this temperature is approximately 400° C., whereas for iron steel material, it reaches 1000 to 1200° C., and accordingly, the tool's material is required to have high temperature strength, toughness, and wear resistance which also allows the material to be welded to plastically flow in this temperature range. This is an issue common to tools used in FSW, FSJ (friction point joining), and friction stirring-applied technology.
Furthermore, a material used for a friction stir welding tool and a hot working tool is required to have wear resistance and defect resistance, and hence not only strength and hardness but also toughness. While W- and Mo-based heat-resistant alloys are mentioned as a heat-resistant material that has been proposed, the inventors have also found that, by adding TiCN to Mo, an alloy which presents an excellent high temperature characteristic is obtained, and as a result of diligent development, by adjusting the amount of TiCN added, the inventors has succeeded in developing a material which is balanced in hardness and strength, and toughness (see patent document 1).
On the other hand, an application in which W- and Mo-based heat-resistant alloys are used as a tool material is often a case in which an iron-based material is assumed as a workpiece, and carbon steel and stainless steel have high deformation resistance, in particular, and accordingly, are regarded as a less workable material. When an iron-based material is subjected by a tool to hot plastic working, the tool in use reaches a temperature around 1000° C., and when a tool with a Mo-based matrix is used, Fe mainly contained in the material to be processed reacts with Mo mainly contained in the tool, and a Fe—Mo-based intermetallic compound may be formed on a surface of the tool. Inter alia, Fe7Mo6 (μ phase) is known to have a hard and brittle nature (see Non-Patent Documents 1 and 2), and when it is formed on a surface of the tool this intermetallic compound phase drops and can thus be a cause of wearing the tool in an increased amount, and accordingly, when an iron-based material is a workpiece, a composition which does not form an intermetallic compound phase is more desirable.
For this, it can be seen from a phase diagram that W similarly known as a refractory material does not form an intermetallic compound in the temperature range in which the tool is used, and using a W-based alloy can prevent an intermetallic compound phase from being formed even when a workpiece is an iron-based material.
As a tool for a friction stir welding a refractory material, a W-based alloy has already attracted attention, and a W—Re alloy and a composite material with a hard material, or a W—Re material (see patent document 2), W-PcBN (see patent document 3) and the like have been developed. Furthermore, friction stir welding tools have been developed of a Co based alloy (patent documents 4 and 5), a W—TiCN alloy (patent document 6 and nonpatent document 3), a Ni based superalloy (patent document 7), an Ir alloy (patent document 8) and silicon nitride (patent document 9).